Gravity Probe B to Test Einstein

A NASA spacecraft designed to test two important predictions of Albert Einstein's
general theory of relativity is set to launch from Vandenberg Air Force Base, Calif.,
at 1:09 p.m. EDT, April 17.

NASA's Gravity Probe B mission, also known as GP-B, will use four ultra-precise
gyroscopes, orbiting the Earth in a unique satellite, to experimentally test two
extraordinary predictions of Einstein's 1916 theory that space and time are
distorted by the presence of massive objects. The two effects being tested are: The
geodetic effect, the amount by which the Earth warps local spacetime in which it
resides, and the frame-dragging effect, the amount by which the Earth drags local
spacetime around with it as it rotates.

"Gravity Probe-B has the potential to uncover fundamental properties of the invisible
universe, a universe which seems very bizarre and alien to our everyday perceptions
yet one that Einstein tried to show us almost a century ago," said Dr. Anne Kinney,
director of the Astronomy and Physics Division in NASA's Office of Space Science,
Washington. "Testing the key aspects of Einstein's theory, such as GP-B will do, will
provide crucial information to science just as it has already helped America by
pushing technological progress in developing the tools needed for these ultra-
precise measurements," she added

Once placed in its polar orbit of 640 kilometers (400 miles) above Earth, GP-B will
circle the globe every 97.5 minutes, crossing over both poles. In-orbit checkout and
calibration is scheduled to last 40-60 days, followed by a 13-month science-data
acquisition period and a two-month post-science period for calibrations.

To test the general theory of relativity, GP-B will monitor any drift in the
gyroscopes' spin axis alignment in relation to its guide star, IM Pegasi (HR 8703).
Over the course of a year, the anticipated spin axis drift for the geodetic effect is
a minuscule angle of 6,614.4 milliarcseconds, and the anticipated spin axis drift for
the frame-dragging effect is even smaller, only 40.9 milliarcseconds. To illustrate
the size of the angles, if you climbed a slope of 40.9 milliarcseconds for 100 miles,
you would rise only one inch in altitude.

During the mission, data from GP-B will be received a minimum of two times each day.
Earth-based ground stations or NASA's data relay satellites can receive the
information. Controllers will be able to communicate with GP-B from the Mission
Operations Center at Stanford University.

Data will include space vehicle and instrument performance, as well as the very
precise measurements of the gyroscopes' spin-axis orientation. By 2005 the GP-B
mission will be complete, and a one-year period is planned for scientific analysis of
the data.

"Developing GP-B has been a supreme challenge requiring the skillful integration of
an extraordinary range of new technologies," said Professor Francis Everitt of
Stanford University, and the GP-B principal investigator. "It is hard to see how it
could have been done without the kind of unique long-term collaboration that we have
had between Stanford, Lockheed Martin, and NASA. It is wonderful to be ready for
launch," he said.

NASA's Marshall Space Flight Center, Huntsville, Ala., manages the GP-B program.
NASA's prime contractor for the mission, Stanford University, conceived the
experiment and is responsible for the design and integration of the science
instrument, as well as for mission operations and data analysis. Lockheed Martin, a
major subcontractor, designed, integrated and tested the spacecraft and some of its
major payload components. NASA's Kennedy Space Center, Fla., and Boeing Expendable
Launch Systems, Huntington Beach, Calif., are responsible for the countdown and
launch of the Delta II.

The launch from Vandenberg will be broadcast live on NASA Television on the AMC-9
satellite, transponder 9C, located at 85 degrees West longitude, vertical
polarization, frequency 3880.megahertz, audio 6.8 megahertz. Information about launch
events and video will be carried on a NASA website called the Virtual Launch Control
Center at: